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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/57303] [4.7/4.8/4.9 Regression] struct miscompiled at -O1 and above
Date: Tue, 21 May 2013 07:57:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57303

--- Comment #7 from rguenther at suse dot de <rguenther at suse dot de> ---
On Mon, 20 May 2013, glisse at gcc dot gnu.org wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57303
> 
> --- Comment #6 from Marc Glisse <glisse at gcc dot gnu.org> ---
> I wonder if, in addition to fixing the sink pass, we should add an optimization
> like the following (it passes bootstrap+testsuite, but I am not so sure where
> it should go and what it should look like exactly).
> 
> --- gimple-fold.c    (revision 199093)
> +++ gimple-fold.c    (working copy)
> @@ -1174,20 +1174,27 @@ fold_stmt_1 (gimple_stmt_iterator *gsi,
>      if ((commutative_tree_code (subcode)
>           || commutative_ternary_tree_code (subcode))
>          && tree_swap_operands_p (gimple_assign_rhs1 (stmt),
>                       gimple_assign_rhs2 (stmt), false))
>        {
>          tree tem = gimple_assign_rhs1 (stmt);
>          gimple_assign_set_rhs1 (stmt, gimple_assign_rhs2 (stmt));
>          gimple_assign_set_rhs2 (stmt, tem);
>          changed = true;
>        }
> +    /* Remove *p = *p.  */
> +    if (!inplace && TREE_CODE_CLASS (subcode) == tcc_reference
> +        && operand_equal_p (lhs, gimple_assign_rhs1 (stmt), 0))
> +      {
> +        gsi_remove (gsi, true);
> +        return true;
> +      }

The obvious place would be dead store elimination.  But beware that
the above, if not _literally_ being *p = *p can be changing the
effective type of the memory location and thus might be not dead
in terms of type-based aliasing rules (which basically means that
we need to do sth more clever than just removing the store).